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Astron. Astrophys. 358, 759-775 (2000)

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9. Shock

For comparison we recall the results of the action of an interplanetary shock on the cometary tail calculated by Wegmann (1995). The calculation started from model 'slow2'. At t=0 the solar wind conditions were changed to 'fast2'. This corresponds to a shock wave of Mach 2 which propagates with a speed of 357 km/s relative to the upstream flow 'fast2'. After three hours the solar wind conditions were changed back to its original values 'slow2'. In this calculation charge exchange ionisation was included. This has little influence on the overall phenomenology.

The middle panel of Fig. 20 shows that after the initial compression of the tail, which leads to enhanced brightness after 1h, the brightness of the coma drops by about a factor of 2 while the brightness of the far tail remains nearly unchanged. The dimming of the coma is quite in accordance with the increase in dynamic pressure in the shock. The disconnected tail moves downstream with an acceleration of a=6.5 m s-2.

The upper panel of Fig. 20 shows that also the ion content per unit tail length is decreased by a factor of 1.5. This means that the tail is really pushed away by the shock. The shock wave is more efficient than the high speed stream (cf. Fig. 3). The turn of the tail in the shock is more gradually. There is no kink visible in the bottom panel of Fig. 20. This feature distinguishes the shock induced DE from that caused by an HSS.

Fig. 19 shows that the new tail, formed after the inverse shock, points into the original direction while the disconnected tail recedes along the direction of the shocked flow. This leads finally to a constellation where the tail looks 'diagonally split' as observed e.g. at comet Borelly on 24 July 1903 (see Brandt 1982, Fig. 9).

[FIGURE] Fig. 19. The same as Fig. 2 for an interplanetary shock of Mach 2. The last image is after 4.5 h.

[FIGURE] Fig. 20. The same as Fig. 3 for an interplanetary shock.

Fig. 21 shows the tail projected onto the perpendicular xz-plane. After 3 hours the tail is really separated from the coma by a broad dim gap. This gap broadens as the old tail moves with an accelerated motion downstream, and the new tail is build up much slower.

[FIGURE] Fig. 21. The same as Fig. 19 but in a projection onto the perpendicular plane.

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© European Southern Observatory (ESO) 2000

Online publication: June 8, 2000